IL-1 is a primary inflammatory cytokine and has been implicated in mediating both acute and chronic pathological inflammatory diseases. Two functionally similar molecules, IL-1α and IL-1β, are encoded by separate genes (respectively, IL1A and IL1B). The third gene of the family (IL1RN) encodes IL-1 receptor antagonist (IL-1ra), an anti-inflammatory non-signaling molecule that competes for receptor binding with IL-1α and IL-1β. Pairwise comparison of IL-1α, IL-1β and IL-1ra yields <25% identity in each case, yet X-ray crystallography of IL-1β and IL-1ra reveal closely similar folds (Priestle et al. (1989) PNAS USA 86: 9667-967); Vigers et al. (1994) Biol Chem 269: 12874-12879). Structurally, the proteins consist of a single domain of 12 packed β-sheets known as a beta-trefoil. Since most of the packing interactions feature main chain atoms, it has been argued that few invariable amino acid are residues required to produce the IL-1 fold, hence extensive diversification of the coding sequences of the genes has been possible. A very similar fold is achieved in soybean trypsin inhibitor without any detectable sequence similarity. All three proteins bind the only functional signaling receptor for IL-1, the type I IL-1 receptor (IL-1R1) (see Sims et al. (1993) PNAS USA 90: 6155-6159).
IL-1 has been characterized mainly as the product of stimulated monocytes, macrophages and keratinocytes, but important roles have been suggested for IL-1 released from smooth muscle and endothelial cells (reviewed by Ross (1993) Nature 362: 801-9). Signaling through IL-1R1 involves the cytoplasmic Toll-like domain of the receptor (Heguy et al. (1992) J Biol Chem 267: 2605-2609). Functional IL-1 receptors are widely distributed in tissues. It is currently believed that IL-1ra differs from IL-1 in failing to activate the interaction between IL-1R1 and the second receptor component, IL-1 receptor accessory protein, IL-1RacP. This is a transmembrane protein that is a distant relative of IL-1R1, having a similar domain structure, but has no intrinsic affinity for IL-1 (Greenfeder et al. (1995) J Biol Chem 270: 13757-13756; Wesche et al., (1997) J Biol Chem 272: 7727-7731).
The IL-1 gene cluster is on the long arm of chromosome 2 (2q13) and contains at least the genes for IL-1α (IL-1A), IL-1β (IL-1B), and the IL-1 receptor antagonist (IL-1RN), within a region of 430 Kb (Nicklin, et al. (1994) Genomics, 19: 382-4). The maximum separation of the distal genes IL1A and IL1RN has been estimated to be 430 kb by pulse field gel electrophoresis of restriction digests of human genomic DNA (Nicklin, et al. (1994) Genomics, 19: 382-4), and the orientation of the three genes has been determined by sequence analysis of physical clones (Nothwang et al. (1997) Genomics 41: 370-378).
There are more than five million Type II (adult onset) diabetics diagnosed in the United States. Type II disease usually begins during middle age; the exact cause is unknown. In Type II diabetics, rising blood glucose levels after meals do not properly stimulate insulin production by the pancreas. Additionally, peripheral tissues are generally resistant to the effects of insulin. The resulting high blood glucose levels (hyperglycemia) can cause extensive tissue damage. Type II diabetics are often referred to as insulin resistant. They often have higher than normal plasma insulin levels (hyperinsulinomia) as the body attempts to overcome its insulin resistance. Some researchers now believe that hyperinsulinomia may be a causative factor in the development of high blood pressure, high levels of circulating low density lipo-proteins (LDLs), and lower than normal levels of the beneficial high density lipo-proteins (HDLs). While moderate insulin resistance can be compensated for in the early stages of Type II diabetes by increased insulin secretion, in advanced disease states insulin secretion is also impaired.
Insulin resistance and hyperinsulinomia have also been linked with two other metabolic disorders that pose considerable health risks: impaired glucose tolerance and metabolic obesity. Impaired glucose tolerance is characterized by normal glucose levels before eating, with a tendency toward elevated levels (hyperglycemia) following a meal. According to the World Health Organization, approximately 11% of the U.S. population between the ages of 20 and 74 are estimated to have impaired glucose tolerance. These individuals are considered to be at higher risk for diabetes and coronary artery disease.
Obesity may also be associated with insulin resistance. A causal linkage among obesity, impaired glucose tolerance, and Type II diabetes has been proposed, but a physiological basis has not yet been established. Some researchers believe that impaired glucose tolerance and diabetes are clinically observed and diagnosed only later in the disease process after a person has developed insulin resistance and hyperinsulinomia.
Insulin resistance is frequently associated with hypertension, coronary artery disease (arteriosclerosis), and lactic acidosis, as well as related disease states. The fundamental relationship between these disease states, and a method of treatment, has not been established.
Associated with insulin resistance is metabolic syndrome. Metabolic syndrome describes several cardiovascular risk factors that are associated with metabolic and inflammatory abnormalities including insulin resistance and obesity. It is believed that this syndrome may have a genetic component.
Recognizing that the entire IL-1 gene locus is centrally involved in inflammatory disease, we herein provide further detailed IL-1 locus polymorphism, linkage, disease association and functional analysis supporting compositions for detecting genetic identity at the human IL-1 locus and their use for the prediction, diagnosis and therapy of insulin resistance, type II diabetes, obesity, and metabolic disorder.